1. Field of the Invention
This invention relates to a method of bonding ceramics and metal, or bonding ceramics together.
2. Description of the Prior Art
A method of bonding oxide ceramics and copper by heating them in an oxidizing atmosphere (hereafter called copper oxide method) has been known, up to now, as a method of obtaining a bonded member from ceramics and metal (see Japanese patent examined publication No. 58-3999 and Japanese patent unexamined publication No. 59-217689).
In addition, it has been suggested that the following method is used as a method of metallizing sintered ceramics: a method comprising the steps of providing sintered ceramics containing silicides which allow manganese and silicon to react at the temperature lower than the melting point of manganese, forming a Mn-containing metallic layer on the surface of the sintered ceramics, and heating the metallic layer up to the temperature lower than the melting point of manganese to cause a bonding reaction between the manganese and the silicon to occur, in which the Mn-containing metallic layer is, prior to the bonding reaction, brought to intimate contact with the surface of the sintered ceramics, so that the manganese reacts on the silicides at the temperature lower than the melting point of manganese (hereafter called bonding method with Mn-containing metal; see Japanese patent unexamined publication No. 58-204885).
In addition, the following method is also generally used: a method of comprising the steps of coating, particularly on oxide ceramics, metallizing paste such as that containing Mo-Mn powder as the main component and SiO.sub.2 and CaO as additional components, sintering the oxide ceramics in a heating and reducing atmosphere to form a metallic layer thereon, and thereafter performing nickel plating, and brazing or soldering (hereafter called Mo-Mn method).
In addition, the following method is also known: a bonding method using a reaction at interfaces between ceramics and the oxgen-active metals such as Ti, Zr or Nb (hereafter called active metal method). For example, Ti-25%V-25%Cr alloy is used for alumina; the bonding is carried out at the bonding temperature of 1,550.degree.-1,650.degree. C. and in an inert atmosphere such as vacuum or Ar gas.
In addition, it has been also suggested, up to now, that the following adhesives are used for bonding ceramics together in connection with oxide ceramics, nitride ceramics and carbide ceramics: adhesives containing, as the effective components, at least one of sodium fluoride and calcium fluoride, or its mixture with kaolin (hereafter called ceramic adhesives; see Japanese patent unexamined publication No. 58-95668).
In addition, the following method is also known: a method of bonding metal and ceramics by applying the ceramic spraying to the surface of the conventional metallic structural member to increase corrosion resistance and heat resistance (hereafter called thermal spraying method). For example, the thermal spraying method has the following steps: the surface of the metallic structural member is roughened by grit blast; the surface preparation is carried out by the thermal spraying of the bonding metal such as molybdenum metal or Ni-5%Al alloy; and the ceramic spraying is carried out to form a ceramic spraying layer on the surface of the metallic structural member, resulting in the bonding of metal and ceramics
As can be seen from above, various methods have been suggested, up to now, as the method of bonding ceramics and metal or bonding ceramics together. These methods, however, have their characteristic problems, which will be discussed below.
Considering first the above copper oxide method of the prior art, this method has the advantage of giving a good bonding strength by one heating process, but has the following problems: the heating in the oxidizing atmosphere would cause a copper oxide film to be produced on the copper surface, the copper oxide film seriously affecting the nature of the copper surface; and the heating up to a high temperature would cause the copper itself to be deformed, thereby requiring surface finish after-treatment. Furthermore, this method encounters the serious problem described below. In the case that this method is followed by the step of bonding the oxide ceramics to a metallic structural member through the thus obtained metallized surface, the use of the brazing near about 800.degree. C. would cause cracking at the ceramics side because of the difference in coefficient of thermal expansion between the ceramics and the copper layer. Accordingly, there is no other way, in performing bonding to the metallic structural member, than to use the soldering below about 300.degree. C., which soldering is poor in strength. Consequently, the resultant total bonding strength would become poor, and the valuable high heat resistance of the ceramics would not sufficiently be effective because of the low heat resistance of the soldering.
Considering next the above bonding method with Mn-containing metal of the prior art, this method has the following drawbacks: the Mn-containing metal layer must be in intimate contact with the ceramics during heating; the ceramics to be bonded must contain the silicides capable of reacting on the Mn-containing metal layer, i.e. SiO.sub.2 and Si.sub.3 N.sub.4 ; and thus the ceramics to be bonded are limited to the specific kinds. Furthermore, this method encounters the following problems: the heating requires the simultaneous pressurization, requiring complicated jigs and/or equipments; and since the silicides capable of reacting on the Mn-containing metal layer must be contained in the ceramics side, the characteristics of the ceramics are often degraded and the kind of the ceramics is limited to the specific ones.
Considering next the above Mo-Mn method, this method has the following drawbacks: two heating processes are needed for the sintering and the brazing or soldering; the nickel plating must be carried out between the two heating processes; and thus poor productivity is expected and complicated schedule control is required. Furthermore, this method encounters the following problems: complicated schedule of sintering-plating-brazing or soldering is required; and there remains a heterolayer in which Mo and/or W powder are mixed with brittle glassy material having a relatively low melting point, such as SiO.sub.2 or CaO.
Considering next the above active metal method of the prior art, this method has the following drawbacks: the high bonding temperature limits possible equipments and causes low productivity; and the high melting point of the brazing filler metal to be used for bonding further limits possible equipments and further reduces productivity. Furthermore, this method encounters the following problems: the high bonding temperature enlarges the difference in heat expansion between the ceramics and the metallic surface; and thus it is inevitable that the total bonding strength becomes poor.
Considering next the above ceramic adhesives of the prior art, this method has the advantage of a high bonding strength after adhesion, but the following drawbacks: the bonding surface becomes the semi-molten state during the heating and bonding, and thus it is difficult to maintain the total dimensional accuracy of the bonded two ceramics. Stating in detail, the effective components of the adhesives are rapidly diffused to the ceramics to be bonded, and the semi-molten glass layer appears at the bonding boundary between the two ceramics. Consequently, it is difficult to maintain the total dimensional accuracy of the two ceramics, and to obtain the uniform boundary layer over the whole of the bonding surface.
Considering next the above thermal spraying method, this method has the following problems: since the bonding mechanism between the metallic structural member and the ceramic spraying layer is the mechanical bonding based on the anchor effect, the bonding strength is poor, and peeling would be caused by the shearing stress which appears at the bonding boundary because of the difference in coefficient of thermal expansion between the metallic structural member and the ceramic spraying layer; and thus the possible temperature range in use is limited, and the life is reduced. For example, the bonding region derived from the thermal spraying of molybdenum metal or Ni-5%Al uses the mechanism, to increase the bonding strength, of the combination of the roughened metallic surface and the sublimation of the oxide film of the molybdenum metal at a relatively low temperature, or uses the mechanism, to increase the bonding strength, of the exothermic reaction under alloying of the Ni-5%Al. These mechanisms are the mechanical bonding based on the anchor effect, resulting in that the bonding strength between the metallic structural member and the ceramic spraying layer is, at maximum, lower than 200-250 kg/cm.sup.2, and the ceramic spraying layer might peel off because of the thermal load and the working stress in use.